I heard stations that for sure are not on 1.8 or 3.5MHz band. This did not depend on signal levels and therefore is not an overloading or nonlinearity issue. So I did search frequencies that produre spurious responses. I used a professional HP signal generator and found out that RSP2 converts signals also with local oscillator 3rd harmonic. Amazing is that this spurious conversion is very effective and only 20dB below the wanted conversion.

This makes RSP2 useless.

Spurios responses are not specified. Is there any way to improve this (without external filters) ? Maybe this is "normal" performance for RSP2 ?? If so SDR radios are as good as conventional radios in year 1910.

I think I've seen this documented somewhere. There are responses based on odd harmonics of the sampling frequency/LO. Just like the 'image' response on a conventional superhet receiver, there is no way to get rid of them without using front-end filtering. The RSP's have filters, but the lowest one covers DC up to 12 MHz, so it includes the unwanted responses when you are tuning the lower bands.This exposes the dilemma facing all SDR designers - leave the front end open so that they can brag about a 10 MHz wide spectrum display or narrow the filters to get rid of the unwanted responses and lose the big waterfall? I find a huge spectrum display a novelty but ultimately I always go back down to a more useful 200 KHz as it makes it easier to see what's what. With that in mind, I use homebrew narrow filters for my bands of interest. ATM I have a VLF filter (0 to 100 KHz), an LF filter (0 to 500 KHz), a MW bandpass filter (500 KHz to 1.7 MHz) and another one for the 160m band (1.8 to 2.0 MHz).All these bands are now clean and very nice to use. For freqs above this (up to 30 MHz) I have a basic HPF that starts roughly at 2 MHz.

It's not 'SDR' radios that are the problem, it's the current fashion for spectrum displays. As well as the RSP2 I have a 'conventional, knob-tuned' SDR receiver, but as it does not drive a spectrum display it does not need to use wide filtering so it does not suffer spurii. IIRC, it uses track-tuned filters for HF and another 18 bandpass filters for VHF and UHF.

It looks like the IQ mixers are not analog multipliers like in old Motorola MC1496 circuit but instead IQ mixers are working in switched mode. So the LO is like a square wave. The spectrum of that includes odd harmonics and the only way to get rid of spurious conversion products is to filter input signals at 3rd harmonic with a low pass filter. This can be done ofcourse with external filters. For 1.8 ... 2MHz filter attenuation at about 3x1.8MHz = 5.4MHz should be 50dB. This figure is based on 70dB dynamic range and the measured -20dBC spurious conversion for 3rd harmonic.

RSP2 price tag is more than 200€. In my opinion this kind poor performance should be mentioned in specifications. And including few LP filters inside RSP would have not costed more than 1to 2£.

Andy, as you seem to be experienced with external filtering, can you possibly recommend a solution for me:

For listening MW and 160m I have a Double-KAZ antenna, with about 1 kohm impedance, symmetric. It's fed to the RSP2's Hi-Z port by using 12 meters of 450 ohms ladder line.

I'd like to add an external low-pass filter for cutting at let's say 1.9MHz. Preferably by using some off-the-shelf solution. But all the available gear (like Mini-Circuits stuff) seem to be for 50 ohms unbalanced.

Jukka - if you want to use off-the-shelf filters, you will need to transform to 50 ohms on the input side and then back up to 1k to feed the RSP. Not ideal. A better solution would be to download an RF package (something like RFSim99) and use it to design your own filter for a 1000 ohm system. All you do is enter the filter type (Chebyshev or Butterworth) HP or LP, roll-off frequency and impedance and press GO. It'll give you a circuit diagram and will also simulate the response for you. Even I can use it!I've just thought - your system is balanced, so either you'll need to do a balanced filter or transform to unbalanced on the input to the filter (use a 1:1 balun) and back to balanced on the output (another balun).RFSim is quite old, but it still works, even on my W8 laptop.

Just one more thought re the spurious responses. If you think about a conventional receiver, you don't know about all the unwanted signals until you happen to tune across one. So you tend to think the receiver is virtually free of such things.When you can see a wide chunk of spectrum, all the nasty stuff is laid out before you and you think the receiver is a sprog box! Good luck with it.